The present invention relates to a capacitor and a method for fabricating the same, more specifically to a capacitor using a high dielectric constant film or a ferroelectric film as the capacitor dielectric film, and a method for fabricating the same.
Capacitors are used in various applications of the field of electronic devices, such as decoupling capacitors for suppressing voltage noises and voltages fluctuations generated across power bus lines, as storage capacitors in semiconductor devices, such as DRAMs, FeRAMs, etc., and as active tunable elements in microwave device application, etc.
As the dielectric films of the capacitors used in these applications, various dielectric materials are proposed, and high dielectric constant and ferroelectric materials of perovskite-type oxides and compounds containing a pyrochlore structure are also studied.
Here, it is known that the film orientation of perovskite-type oxide and of compounds containing a pyrochlore structure as the dielectric, profoundly impacts the electrical properties of the capacitor.
For example, as for BST (BaSrTiOx: barium strontium titanate) film, the (100) oriented film can have high voltage dependency of the capacitance value. Accordingly, in a case that BST film is used in voltage tunable applications, etc., it is preferable that the (100) oriented film is used. However, the (100) oriented film has high interface state density in the interface with platinum (Pt) used as an electrode material. Accordingly, electrons tend to be trapped in the electrode interface, and the leakage current in strong electric fields has high time dependency unsuitably to be used in applications requiring the characteristic to be stable for a long period of time.
On the other hand, as for BST film formed by MOCVD, the (110) oriented film has the interface state density in the Pt/BST interface lower than the (100) oriented film. By using the (110) oriented film, the time dependent leakage current can be suppressed or precluded. Thus, it is preferable that the (110) oriented film formed by MOCVD is used in applications required the characteristic to be stable for a long period of time.
Capacitors using dielectric films having perovskite structure and pyrochlore structure have the electric characteristics are degraded by the exposure to hydrogen ambient. In this case as well, the film orientation dependency of the dielectric films are found. That is, when the (100) oriented films are subjected to, e.g., forming gas annealing for improving characteristics of transistors after the capacitors have been formed, the electric characteristics are much affected, such as the leakage current being increased, the capacitance value being decreased, etc. (111) oriented films and (110) oriented films, however, can suppress such electric characteristics degradation. This will be because the (100) oriented film has fine structure and grain boundaries extended perpendicular to the surfaces of the film, whereby hydrogen more tends to diffuse in the film in comparison with the (111) oriented film and the (110) oriented film.
As described above, dielectric films having perovskite structure and pyrochlore structure are strongly dependent on orientation of the films. Accordingly, when capacitors are formed of such dielectric films, it is preferable to select orientations of the films in accordance with applications in which the films are to be used.
As described above, dielectric films having perovskite structure and pyrochlore structure have the characteristics highly dependent on orientations of the films. When one film orientation is selected so as to optimize one characteristic (e.g., voltage tunability of the capacitance value), it cannot help sacrificing another characteristic (e.g., leakage current characteristic).
A capacitor which is characterized optimum for one application must have the characteristics improved in other viewpoint so as to further improve device characteristics. Accordingly, a capacitor which permits various characteristics provided by the respective film orientation of the dielectric film to be concurrently improved has been required.
An object of the present invention is to provide a capacitor which can concurrently improve various characteristics dependent on film orientations of the dielectric films, and a method for fabricating the same.
According to one aspect of the present invention, there is provided a capacitor comprising; a lower electrode; a capacitor dielectric film formed on the lower electrode and including at least two dielectric films whose film orientations are different from each other and which are spaced from each other by a floating electrode; and an upper electrode formed on the capacitor dielectric film.
According to another aspect of the present invention, there is provided a capacitor comprising: a lower electrode; a first dielectric film formed on the lower electrode; a first floating electrode formed on the first dielectric film; a second dielectric film formed on the first floating electrode and having a film orientation different from that of the first dielectric film; and an upper electrode formed on the second dielectric film.
In the above-described capacitor it is possible that the capacitor further comprises a second floating electrode formed on the second dielectric film; and a third dielectric film formed on the second floating electrode and having a film orientation different from that of the second dielectric film.
According to further another aspect of the present invention, there is provided a capacitor comprising: a lower electrode; a first dielectric film formed on the lower electrode; a floating electrode formed on the first dielectric film; a second dielectric film formed on the floating electrode and formed of a dielectric material different from that of the first dielectric film; and an upper electrode formed on the second dielectric film.
According to further another aspect of the present invention, there is provided a method for fabricating a capacitor comprising the steps of: forming a first dielectric film on a lower electrode; forming a first floating electrode on the first dielectric film; forming on the first floating electrode a second dielectric film having a film orientation different from that of the first dielectric film; and forming an upper electrode on the second dielectric film.
In the above-described method for fabricating the capacitor it is possible that the method further comprises, after the step of forming the second dielectric film, the steps of: forming a second floating electrode on the second dielectric film; forming on the second floating electrode a third dielectric film having a film orientation different from that of the second dielectric film.
As described above, in the capacitor according to the present invention including the capacitor dielectric film of high dielectric constant film or ferroelectric film, the capacitor dielectric film is formed of at least 2 dielectric films having film orientations different from each other, whereby various characteristics which have dependency on film orientations of the dielectric films can be simultaneously improved.